A group of sterically encumbered pen-substituted acenaphthenes have been prepared, containing tin moieties at the 5,6-positions in 1-3 ([Acenap(SnR3)(2)], Acenap = acenaphthene-5,6-diyl; R-3 = Ph-3 (1), Me-3 (2); [(Acenap)(2)(SnMe2)(2)] (3)) and phosphorus functional groups at the proximal pen-positions in 4 and 5 ([Acenap(PR2)(PR2)-((PPr2)-Pr-i) R-2 = Ph-2 (4), Ph(Pr-i) (5)). Bis(stannane) structures 1-3 are dominated by repulsive interactions between the bulky tin groups, leading to pen-distances approaching the sum of van der Waals radii. Conversely, the quasi-linear C-ph-P center dot center dot center dot P three-body fragments found in bis(phosphine) 4 suggest the presence of a Ip(P)-sigma*(P-C) donor-acceptor 3c-4e type interaction, supported by a notably short intramolecular P center dot center dot center dot P distance and notably large J(pp) through-space coupling (180 Hz). Severely strained bis(sulfides) 4-S and 5-S, experiencing pronounced in-plane and out-of-plane displacements of the exocyclic pen-bonds, have also been isolated following treatment of 4 and 5 with sulfur. The resulting nonbonded intramolecular P center dot center dot center dot P distances, similar to 4.05 angstrom and similar to 12% longer than twice the van der Waals radii of P (3.60 angstrom), are among the largest ever reported peri-separations, independent of the heteroatoms involved, and comparable to the distance found in 1 containing the larger Sn atoms (4.07 angstrom). In addition we report two metal complexes with square planar [(4)PtCl2] (4-Pt) and octahedral cis-[(4)Mo(CO)(4)] (4-Mo) geometries. In both complexes the bis(phosphine) backbone is distorted, but notably less so than in bis(sulfide) 4-S. All compounds were fully characterized, and except for bis(phosphine) 5, crystal structures were determined.